As known in the art, structures may be bonded together by means of linear friction welding. In such a process, a surface on one of the structures is contacted (interfaced) to a surface on the other structure. The interfacing surfaces typically have complementary features, i.e. similar lengths and similar widths. The two parts are rubbed together, in a back and forth, in a linear type oscillatory manner. The axis of the oscillation is typically roughly aligned with the longitudinal (lengthwise) axis of the interface, i.e. end to end. As the parts are rubbed, compressive force is applied to place the interface under high pressure. At the interface, frictional heat is generated and material from each part changes to a solid plastic state. Some of this material flows out from between the parts (flash flow), resulting in gradual decrease in the thickness, i.e. the dimension in the direction in which pressure is applied (the dimension perpendicular to the interface) to the part. When the process is terminated, flash flow ceases, and at the interface, the remaining plastic state material cools and forms a solid state bond of the two parts.
However, a problem exists with this process in that the bond is usually incomplete, i.e. defective, at the ends of the interface. The nature of the defect is lack of bonding in the shape of a notch. It occurs in part because the ends of the interface, roughly on the axis of oscillation, are alternatingly exposed to ambient during each oscillation cycle. While exposed, the end is not rubbed and therefore not frictionally heated. Thus, as a result of the alternating exposure, the ends are only alternatingly heated and the temperature of the ends does not get high enough to produce complete bonding.
Efforts have focused on developing processes which ensure that the defect does not form within the outline of the final shape of the product. In the fabrication of original equipment, part geometries can be oversized so that the defects that form are located outside the outline of the final product. The defects are then removed as the product is machined down to its final shape. However, in repair situations, a damaged portion is removed, but the remaining portion is already at its final shape and dimension, and therefore, an oversized geometry is not a viable alternative.
One of the numerous applications for linear friction welding is that of attaching blades (airfoils) to a rotor and thereby forming an integrally bladed rotor (IBR). In such an application, a base surface on the airfoil is interfaced to a slightly elevated surface on the rotor. However, without preventative measures, the bond risks being defective at the airfoil edges, because the airfoil edges are situated at the ends of the interface, roughly on the oscillation axis. As a result, the airfoil edges are alternatingly exposed to ambient and only alternatingly heated during oscillation and the edge temperature does not get high enough to produce complete, adequate bonding. Although the defect may not constitute a crack per se, it could initiate a crack during engine operation, and thus its presence in an IBR is unacceptable.
In the prior art approach for preventing defects at the edges for IBR repairs, the damaged portion of an airfoil under repair is removed, e.g. by removing a longitudinal section, and flanges or collars are provided around the edges of the remaining portion. The flanges or collars are supported by a pair of jaws gripping the undamaged remaining airfoil section (or stub). The collars have a shape closely matching the shape of the undamaged airfoil stub. Similar flanges and jaws are provided to grip a replacement airfoil portion. The collars around each portion prevent the airfoil edges of the other portion from being alternatingly exposed to ambient and sufficient heat is generated to achieve bonding. Any defects formed reside in the collar regions and are machined away after joining. With this prior art approach, the (relatively) massive jaws holding the collars and airfoils can cause damage to both the remaining stub and replacement airfoil due to inaccuracies in shape mismatch between the collars and blades and due to the relatively soft nature of titanium alloys, prime candidates for this application.
These and other issues were solved by the process taught in commonly owned U.S. Pat. No. 5,865,364 to Trask et al. and hereby incorporated herein in its entirety. Improvements to the processes taught in the above-mentioned patent are the subject of this invention.